1. The Field of the Invention
The present invention is related to noninvasive systems and methods which are used to monitor the physiological condition of a patient's circulatory system. More particularly, the present invention is related to an enhanced noninvasive system and method for monitoring a patient's arterial oxygen saturation, and which also provides continuous measurement of blood pressure.
2. The Background Art
The proper utilization of many lifesaving medical techniques and treatments depends upon the attending physician obtaining accurate and continually updated information regarding various bodily functions of the patient. Perhaps the most critical information to be obtained by a physician, and that which will often tell the physician a great deal concerning what course of treatment should be immediately instituted, are heart rate, blood pressure, and arterial oxygen saturation.
In settings such as operating rooms and in intensive care units, monitoring and recording these indicators of bodily functions is particularly important. For example, when an anesthetized patient undergoes surgery, it is generally the anesthesiologist's role to monitor the general condition of the patient while the surgeon proceeds with his tasks. If the anesthesiologist has knowledge of the patient's arterial oxygen saturation, heart rate, and blood pressure, the general condition of the patient's circulatory system can be assessed.
Arterial oxygen saturation (abbreviated herein as S.sub.a O.sub.2) is expressed as a percentage of the total hemoglobin in the patient's blood which is bound to oxygen. The hemoglobin which is bound to oxygen is referred to as oxyhemoglobin. In a healthy patient, the S.sub.a O.sub.2 value is above 95% since blood traveling through the arteries has just passed through the lungs and has been oxygenated. As blood courses through the capillaries, oxygen is off-loaded into the tissues and carbon dioxide is on-loaded into the hemoglobin. Thus, the oxygen saturation levels in the capillaries (abbreviated herein as S.sub.c O.sub.2) is lower than in the arteries. Furthermore, the blood oxygen saturation levels in the veins is even lower, being about 75% in healthy patients.
Importantly, if the patient's arterial oxygen saturation level is too high or too low, the physician may take action such as reducing or increasing the amount of oxygen being administered to the patient. Proper management of S.sub.a O.sub.2 is particularly important in neonates where S.sub.a O.sub.2 must be maintained high enough to support cell metabolism but low enough to avoid damaging oxygen-sensitive cells in the eye and causing impairment or complete loss of vision.
Blood pressure monitoring includes at least three values which are of interest to a physician. First, the systolic pressure is the high pressure generated in the arteries during contraction (or systole) of the left ventricle of the heart. Second, the diastolic pressure is the pressure maintained in the arteries during relaxation (or diastole) of the left ventricle. Due to the elastic nature of the walls of the arteries, the diastolic pressure is above zero but less than the systolic pressure.
A third value of interest to a physician is the mean arterial pressure. The mean arterial pressure may be simply described as the arithmetic average of all the blood pressure values between, and including, the systolic and diastolic pressures. In addition to the just mentioned three discrete blood pressure values, a physician is also interested in obtaining the blood pressure waveform. As is well known, patients having identical systolic and diastolic values may have very different mean arterial pressures and their blood pressure waveforms may be dramatically different. Having the blood pressure waveform at hand allows the physician to more accurately assess the patient's condition.
Blood pressure is generally measured quantitatively in millimeters of mercury (mmHg) referenced against atmospheric pressure (about 760 mmHg). Thus, in a normal person the blood pressure may be 120 mmHg above atmospheric pressure during systole and 70 mmHg above atmospheric pressure during diastole. Such values are commonly recorded as "120 over 70" (120/70).
Continuous monitoring of arterial oxygen saturation levels (S.sub.a O.sub.2) and arterial blood pressures each present unique problems.
One method of determining S.sub.a O.sub.2 is to withdraw blood from an artery and analyze the same to determine the amount of oxyhemoglobin present. While in vitro analysis provides the most accurate blood gas determinations, the disadvantages of drawing a blood sample each time an S.sub.a O.sub.2 determination is desired by the physician is readily apparent. Significantly, even in the operating room in vitro S.sub.a O.sub.2 determinations may take up to several minutes. Since nerve cells deprived of sufficient oxygen begin to die in a matter of minutes, the time taken to obtain the results of an in vitro S.sub.a O.sub.2 analysis may seriously compromise patient safety.
Particularly in the case of a patient undergoing routine surgery, the difficulties of withdrawing blood samples throughout the surgical procedure for S.sub.a O.sub.2 determinations is generally too great to be adopted as a general practice. Still, monitoring of S.sub.a O.sub.2 during all surgeries where general anesthesia is used and in intensive care units is expected to have a significant positive effect on the well -being of patients. Thus, past efforts have been directed to providing noninvasive systems and methods for determining arterial S.sub.a O.sub.2.
The term "oximetry" has been adopted in the art to refer to noninvasive apparatus and methods for determining blood oxygen saturation levels. Previously available oximetry systems make use of the fact that the absorption characteristics of different blood components, namely, HbO.sub.2 and Hb and also referred to as the coefficient of extinction, differ depending upon which wavelength of light (e.g., infrared or visible portions of the spectrum) is being used.
Thus, previously available noninvasive oximetric systems impinge at least both visible and infrared light upon a body part, such as a finger, and then estimate the SO.sub.2 level using the relative proportions of visible and infrared light which was transmitted or reflected. Undesirably, such systems inherently include some inaccuracy, which increases to a substantial error for low (50-70%) SO.sub.2 levels, due to, among other things, the inclusion of capillary blood as well as arterial blood in the reading.
In an effort to improve the accuracy of the SO.sub.2 values obtained using only two wavelengths of light, rather than the bulky and expensive ear oximeter previously available, which impinged light of eight different wavelengths on the body part, other apparatus have utilized the pulsatile component of the transmitted or reflected light beam to distinguish variations in the detected intensity of the light beam which are due to changes in blood components from other causes. Generally referred to as pulse oximetry, using the pulsatile signal modulating the light beams for S.sub.a O.sub.2 estimate provides a significant improvement in accuracy over nonpulse oximetry systems yet still does not distinguish between arterial blood oxygen saturation and capillary blood oxygen saturation.
The previously available systems and methods of monitoring blood pressure also all have a variety of disadvantages. The most commonly performed method, the auscultatory sphygmomanometer method (utilizing a pressure cuff, mercury manometer, and a stethoscope), often provides reasonable estimates of systolic and diastolic blood pressure. But the method does not provide any information concerning the mean blood pressure or the pressure waveform. Moreover, a trained professional must take one or more minutes to carry out the method and even then may be unsuccessful.
Arterial catheterization provides very accurate blood pressure measurements and waveforms in critical care situations. The extreme invasiveness and the risks of catheterization, including infection, thrombus formation, hemorrhage, and cerebral embolization precludes the method from being routinely used on patients.
In an attempt to provide noninvasive blood pressure monitoring devices, several methods have been suggested in the past. Devices incorporating a constantly inflated finger cuff which tracks the pressure changes within the finger disadvantageously may cause pain to the patient, interference with the pressure measurement, and/or tissue damage.
In an effort to avoid the disadvantages of using a constantly inflated pressure cuff, various devices utilizing photoplysmography have been introduced. While such devices utilize a light beam directed at the finger, or other body part, to sense changes in blood vessel volume in order to determine changes in pressure and thus avoid the use of a constantly inflated pressure cuff, such devices still suffer from inaccurate readings, particularly when determining the diastolic pressure, and such devices still cannot provide an accurate representation of the arterial pressure waveform.
In view of the disadvantages and drawbacks of the previously available apparatus and methods, it would be an advance in the art to provide a system and method for noninvasively measuring arterial blood oxygen saturation levels while minimizing the effect of capillary oxygen saturation on the measurement. It would be another advance to provide a system for measuring both arterial oxygen saturation levels and blood pressure using no more hardware than necessary to measure oxygen saturation. It would also be an advance in the art to provide a system and method for noninvasively measuring blood oxygen saturation levels and blood pressure which minimizes contact with, and the pressure applied to, the body of the patient. It would be a further advance in the art to provide a system for noninvasive blood oximetry or blood pressure monitoring which may be applied to any one of several parts of the patient's body.
It would also be an advance in the art to provide both a method and system for blood oximetry and blood pressure monitoring which may be implemented using little specialized hardware. It would be yet another advance in the art to provide a noninvasive blood pressure monitoring system and method which can provide systolic, diastolic, and mean arterial pressure measurements as well as an accurate representation of the pressure waveform. Still another advance in the art would be to provide a noninvasive system and method for measuring arterial blood oxygen saturation levels which enhances the arterial contribution and reduces the influence of the capillary contribution to the oxygen saturation measurement.